Thank you wizard.

Yes, I would buy the CPL170 just as a keyboard and play through an external sound generator: Ivory, as you said, Vintage D or else.

But I considered also to use the internal CLP170 speakers to amplify the sound generated by the PC... but your answer made me disappointed! I thought there was no problem to amplify external source!

Yeah, I was pretty disappointed back then, too. I thought that as the speakers and amp have quite good specs this should work well. It works, but it does not work well. The fact really is, that every digital piano manufacturer tries very hard to accomplish a sound output through the speakers that comes close to an acoustic piano. Therefore they have to tailor the speakers and amplifier to their internal sound generator.
As I mentioned in my last post I tried other digital pianos, too. Those were all recent models and they all didn't work well with external sources (I used Ivory). Amongst the ones I tested were Kawai CA-93 and CA-63 and the new Roland HP series. The sound output via speakers with external source was really disappointing. The internal sound generator was always far ahead. Whilst this isn't a real problem for the Kawai CA series, this is a problem for the CLP 170, because the internal sound generator isn't really good.

yeah... thanks again. What a pitty!